Version: 2.0, 3.2, 4.0, 4.01
Compability: All
The DOCTYPE tag specifies the Document Type Definition (DTD) for HTML. This sets the rules for the grammar and syntax of the HTML language. While his tag is not mandatory, the W3C (World Wide Web Consortium), who set the standard for HTML 4.01, strongly recommends that you always include it in your code.
This tag can only be placed as the first line of the HTML code. It is the only tag that can proceed the HTML tag.
HTML 4.01 TRANSITIONAL//EN">
<html>
...
</html>